This invention relates generally to shielded spark plug boot assemblies and more specifically to shielded spark plug boot assemblies having an electrically conductive housing or shell that is grounded to the metal shell of the spark plug.
It is already known from U.S. Pat. No. 4,497,532 granted to Michael J. Bezusko; Daniel P. Liska and Joseph A. McGee Feb. 5, 1985 to provide a heat shielded, spark plug boot assembly 10 comprising an outer metal shell 46 that has ears 54 or 64 that engage the metal shell 24 of a spark plug for grounding the electrically conductive outer metal shell 46. This patent also shows a heat shielded, spark plug boot assembly 70 comprising a laminate tube 72 that has a circumferential bead 74 for this purpose.
Heat shielded, spark plug boot assemblies of the above type that have grounding ears on the bottom of the outer metal shell have been manufactured and used successfully by General Motors Corporation for several years. Even though these assemblies are entirely satisfactory from several standpoints, the assemblies require a relatively high engagement force because of the nature of the grounding ears. These ears need to engage the metal shell of the spark plug with a relatively high interference fit to compensate for their low spring force characteristics.
It is also already know from U.S. Pat. No. 3,128,139 granted to Stanley E. Estes Apr. 7, 1964 to provide a spark plug shield 10 that includes a leaf spring 29 that is disposed inside an electrically conductive housing 13. The leaf spring 29 engages the metal shell 14 of a spark plug 11 to ground the electrically conductive housing 13 when the shield 10 is plugged onto the spark plug 11.
The spark plug shield of this patent does not require a high engagement force to plug the shield onto the spark plug because of the nature of the grounding leaf spring. However, the leaf spring has a relatively low spring force characteristic and is damaged easily if the shield is not plugged onto the spark plug carefully.
It is also already known, in Europe at least, from a spark plug shield that is used on an Adam Opel Calibra automobile to provide a spark plug shield that includes a coil spring for grounding the electrically conductive metal shell of the shield to the metal shell of a spark plug. This spark plug shield does not require a high engagement force and the coil spring clearly has adequate spring force characteristics to maintain the grounding connection. However, the coil spring is attached to the end of the metal shell and entirely exposed so that the spark plug shield is susceptible to entanglement during shipment.